Throughout the last five years there has been a considerable interest growing among chemists, physicists, and electrical engineers in the development of electrically-conductive organic polymers for use in a variety of applications. Potential applications include lightweight electrical conductors, microwave shields, anti-static devices, photocopying processes, and photovoltaic devices. There are several advantages in using organic polymers versus classical inorganic materials for these applications: organic polymers are by nature lightweight and easily processable, and they offer lower materials costs. For purposes hereof, the term "electrically-conductive organic polymers" refers to polymers whose conductive properties are derived from the conduction band structure of the polymer itself, rather than through the addition or impregnation of a conductor into a polymer substrate. Accordingly, conductive polymeric systems composed of a conductive material (such as powdered graphite or copper fibers) imbedded in an insulating organic polymer matrix or substrate is not considered a conducting polymer within the meaning of the present invention.
To the best of our knowledge, the first example of a highly conductive organic polymer film was demonstrated by researchers at the University of Pennsylvania and reported by Shirakawa et al., Chem. Commun., 1977, p. 578. Their results with chemically doped polyacetylene films greatly stimulated research in this field.
The electrical conductivities of most organic polymers in their virgin state tend to be low; typical values range from 10.sup.-5 to 10.sup.-14 (.OMEGA.-cm).sup.-1. If compared to the conductivities of classical inorganic materials, such as copper (10.sup.6 (.OMEGA.-cm).sup.-1), silicon (10.sup.-5 (.OMEGA.-cm).sup.-1), or quartz (10.sup.-18 (.OMEGA.-cm).sup.-1), most organic polymers would be termed insulators or poor semiconductors. In fact, organic polymers are widely used as electrically-insulating materials in the electronics industry.
A consideration of the band theory as developed for classical inorganic materials is often helpful in describing in very simple terms the insulating nature of a multitude of organic polymers, such as polytetrafluoroethylene (Teflon), polyethylene, polystyrene, and like materials. The low conductivities exhibited by most polymers can be envisioned as resulting from the presence of filled valence electron bands with large energy separations between the valence or highest occupied molecular orbital (HOMO) and conduction or lowest unoccupied molecular orbital (LUMO) bands in these materials. The HOMO-LUMO energy gaps in polyethylene and similar polymers are generally greater than about 3 eV. Classical inorganic materials with valence-conduction band gaps of this magnitude exhibit electrically insulating behavior.
However, presently there are a number of different highly conductive (conductivities around 10.degree. (.OMEGA.-cm).sup.-1) organic polymers that have been well documented in the open literature. The earliest recognized and most extensively studied of these is polyacetylene, (CH).sub.x, doped with electron-accepting or donating reagents. Work has been done in this area by Shirakawa et al.; Chiang et al., Phys. Rev. Lett., 1977, 39, 1098; Park et al., J. Chem. Phys., 1980, 73, 946; and Chiang et al., Ber. Bunsenges. Phys. Chem., 1979, 83, 407. Shacklett et al., Synthetic Metals, 1979, 1, 307 has worked with doped poly-p-phenylene and others have investigated poly-p-phenylenesulfide, poly-p-phenylenevinylene, polypyrrole and poly-thienylene. A common feature of all these polymers is a molecular structure possessing some degree of .pi.-electron conjugation along the polymer chain. For poly-p-phenylenesulfide, it is postulated that empty sulfur d-orbitals participate in .pi.-conjugation with the phenylene .pi.-system. Although the exact mechanism of charge transport in these doped polymers is still under great debate, it is generally recognized that some degree of .pi.-conjugation in the polymers is a prerequisite to high conductivity.
The polymers mentioned above all exhibit low electrical conductivities (e.g., 10.sup.-9 (.OMEGA.-cm).sup.-1 for cis-polyacetylene) before they are chemically treated or "doped" with appropriate electron-accepting or donating reagents. For purposes hereof, the term "doped" in this art refers to the formation of charge transfer complexes between suitable organic polymers and appropriate electron-accepting or electron-donating reagents. This usage of the term is to be distinguished from the usage associated with the semiconductor art which pertains to the positional substitutions of certain atoms for other atoms, as in "doped" inorganic semiconductors. Reaction of suitable organic polymers with electron-accepting reagents results in transfer of electron density from the .pi.-orbitals of the polymers to the acceptor. Similarly, reaction with an electron donor causes addition of electron density to the .pi.-system of the polymers from the donor. For purposes hereof, a "doped polymer" is therefore a polymer which has undergone changes in its .pi.-system electron density through the formation of charge-transfer complexes by reaction of the polymer with suitable electron-acceptor or electron-donor reagents. Such partial oxidation or reduction of polymers upon doping with appropriate reagents is believed to be responsible for the greatly enhanced electrical conductivities displayed by these polymer systems.
Shirakawa et al. and Park et al. have disclosed that a variety of Lewis acids and bases are effective dopants for enhancing the conductivity of polyacetylene. Oxidants such as iodine, bromine, and AsF.sub.5 have been employed, and they indicate that the dopants remain in the polymer matrix after charge transfer as, for example, I.sub.n.sup.- or AsF.sub.6.sup.- anions. It is also known in the art that polyacetylene can also be reduced with Lewis base alkali metal alloys or sodium naphthalide in tetrahydrofuran; charge transfer results in the inclusion of cations (i.e., alkali metal cations) in the polymer matrix.
There has not been total agreement upon a description of the ways in which the polymer-dopant charge transfer complex manifests itself and influences the charge transport mechanisms in these conjugated polymer systems. For heavily-doped polyacetylene films with conductivity of about 10.sup.2-3 (.OMEGA.-cm).sup.-1), a band theory model seems adequate. In this model, the population of charge carriers in the valence and conduction bands of (CH).sub.x has been altered sufficiently that individual strands of polyacetylene within a polymer film are described as metallic; however, the "metallic" strands are separated by thin regions of interstrand contact characterized by a potential barrier to charge transport. Along the strands, conduction is metallic; between strands, conduction occurs via a thermally-activated process.
For lightly-doped semiconducting (CH).sub.x films with conductivities of about 10.sup.-6 (.OMEGA.-cm).sup.-1, band theory is inadequate in explaining all the charge transport phenomena. Instead, a mechanism involving the formation of charged solitons--or rather localized charged domain walls, akin to organic radicals--has been proposed. In this mechanism, charge transport would occur via thermally-activated hopping along the polymer chain of the domain walls.
Of these conjugated polymer systems, only poly-p-phenylenesulfide (PPS) exhibits favorable fabrication properties as well as favorable thermal and atmospheric stability; PPS can be heat molded. However, PPS doped with AsF.sub.5 is much less stable to the atmosphere and more brittle than virgin PPS. To date, only AsF.sub.5 has been reported as a suitable dopant for PPS. With certain of these polymer systems (PPS, poly-p-phenylene, poly-p-phenylenevinylene) it appears that I.sub.2 and Br.sub.2 are not strong enough oxidants to effect conductivity enhancements. Fabrication difficulties also exist with polypyrrole. Films of this polymer can only be obtained in situ as the monomer is polymerized. Once polymerized, polypyrrole cannot be further processed by solution or melt methods. Polythienylene is quite stable in air and can be doped with I.sub.2, but it exists as an intractable powder.
Although doped polyacetylene exhibits the highest conductivity as well as the broadest range of accessible conductivities (as a function of dopant concentration) of an organic polymer currently known, polyacetylene does not exhibit environmental stability or desirable fabricating properties. More specifically, the major limitations in the practical applications of polyacetylene as an organic conductor are the extreme insolubility of (CH).sub.x in solvents other than concentrated H.sub.2 SO.sub.4, and the chemical instability of (CH).sub.x and its conductive derivatives in the ambient atmosphere and at elevated temperatures. Fabricating films or coatings of (CH).sub.x, after the acetylene has been polymerized, is nearly impossible due to the insolubility of (CH).sub.x and its inability to be molded by heat-pressing techniques. Consequently, fabrication of (CH).sub.x would be necessary in situ, as the acetylene is polymerized. Polyacetylene is also unstable with respect to air oxidation before doping and becomes even more unstable after doping. The chemical degradation of the conductive (CH).sub.x in air or at elevated temperatures is accompanied by a decrease in the electrical conductivity of these materials. Hence, long-term stability of the electrical properties in these systems is difficult to achieve.
Carr et al. in U.S. Pat. No. 4,160,760 disclose a method for interacting Prussian blue with polyacrylonitrile to produce a polymer with enhanced color fastness and electrical properties. While the primary focus of the Carr et al. reference appears to be obtaining a polymer which exhibits a homogeneous color, Carr et al. note that "enhanced . . . conductivity" may also occur. No reason for this speculation and no conductivity measurements or tests were reported, however. It should also be noted that Carr et al. deals exclusively with uncondensed polyacrylonitrile.
For potential applications of conductive organic polymers, it is desirable to develop polymeric systems exhibiting favorable fabrication, solubility, environmental and stability characteristics in addition to electronic structures conducive to the formation of conductive charge transfer complexes with electron acceptors.
It is an object of the present invention to develop a polymer system which exhibits conductivity in the range of semiconductors (from about 10.sup.-10 to about 10.sup.2 (.OMEGA.-cm).sup.-1). It is a further object of this invention to develop a conductive polymer possessing semiconductor properties as well as favorable environmental properties. It is another object of this invention to develop a conductive polymer having favorable environmental properties which can be readily fabricated and processed. It is another object of this invention to develop a conductive polymer system possessing favorable stability and solubility characteristics. It is a further object of this invention to develop a method for fabricating such a polymer system.